add alertmanager ui

handlers/main.yml

+- name: regenerate alertmanager ui
+  command: /usr/share/prometheus/alertmanager/generate-ui.sh
 - name: restart alertmanager
   service:
     name: prometheus-alertmanager

tasks/main.yml

 - name: install alertmanager
   apt:
     pkg: prometheus-alertmanager
+  notify: regenerate alertmanager ui
 
 - name: write alertmanager service config
-  notify: restart alertmanager
+  notify:
+  - regenerate alertmanager ui
+  - restart alertmanager
   template:
     src: prometheus-alertmanager.j2
     dest: /etc/default/prometheus-alertmanager